The use of solid state circuits for eliminating contact arcing in circuit interruption devices has heretofore not proven economically feasible. When switchable circuit elements are employed, some complex additional circuitry is required to turn on the solid state circuit device to immediately divert current away from the separating contacts and then to transfer the current to a voltage controlled device, such as a metal oxide varistor.
The use of a silicon controlled rectifier for diverting current away from separating contacts is given within British Patent Specification No. 1,072,267. The use of a triac having a voltage dependent resistor connected across the gate circuit of the triac is disclosed British Patent Specification No. 1,152,903.
U.S. Pat. No. 3,783,305 describes a logic circuit connected to the control electrode of a thyristor for generating a trigger pulse to the thyristor upon contact separation.
U.S. Pat. No. 4,583,146 entitled "Fault Current Interrupter" in the name of E. K. Howell discloses the use of a positive temperature coefficient element and a varistor connected in parallel across separating contacts. The positive temperature coefficient element is capable of diverting current away from the contacts and over to the varistor by virtue of its temperature responsive properties.
U.S. patent application Ser. No. 681,478 filed Dec. 14, 1984 entitled "Circuit Interrupter Using Arc Commutation" in the name of E. K. Howell utilizes a zener diode in the gate circuit of a solid state switch to turn on the solid state switch when the arc voltage across a pair of separated contacts reaches a predetermined voltage. A capacitor connected in parallel with a varistor rapidly charges to the clamping voltage of the varistor to transfer the current to the varistor and away from the solide state switch.
U.S. patent application Ser. No. 610,947 filed May 16, 1984 entitled "Solid State Current Limiting Circuit Interrupter" utilizes a bi-polar power transistor to switch current away from separating contacts to a metal oxide varistor. The transistor is first turned on by a current pulse provided by a capacitor connected between the transistor collector and base. A saturable core current transformer in circuit with the transistor provides regenerative base drive for the transistor and the transistor turns off as soon as the transformer core becomes saturated. Also disclosed is the use of a field effect transistor, field controlled transistor and gate turn-off devices such as thyristors in place of the bi-polar power transistor.
The purpose of this invention is to provide a rapid means for transferring current from separating contacts to a metal oxide varistor in a short period of time and with a minimum amount of circuit components.